1. Field of the Invention
The present invention relates generally to a system and method for controlling an air/fuel mixture ratio for an internal combustion engine and particularly relates to a system and method which detects the air/fuel mixture ratio from the exhaust gas of the engine to carry out feedback control of the air/fuel mixture ratio on the basis of the detected signal thereof so that the air/fuel mixture ratio of the air fuel mixture supplied to the engine reaches a target air/fuel mixture ratio (so called, stoichiometric air/fuel mixture ratio) on the basis of the detected result of the air/fuel mixture ratio from the exhaust gas.
2. Description of the Background Art
Fuel injection systems using microcomputers have been developed which exhibit excellent performance which is superior to carburetor systems. In addition, the high control accuracy of air/fuel mixture ratios provided by fuel injection systems completely coincide with requirements for technologies for suppressing harmful components in exhaust gas which are strictly regulated.
One of advantages of the fuel injection system is that a great number of components can be miniaturized although control components have multi-functions. One of the other advantages of the fuel injection system is that if a program is prepared and stored according to the desired control functions, its functions can be freely expanded. Program modifications can be made with the computer hardware itself remaining unchanged. In addition, since the data required for control can be stored, optimal system control as obtained in testing laboratories can be achieved without compromise. This results in high performance engine control. (Refer to pages 28 through 40 of "Automotive Engineering" published by Tetsudo Nippon Sha on October, 1985, and pages 108 through 114 of the same title published on January, 1986, and pages 47 through 56 of "Car Electronics" published by Kabushiki Kaisha Okawa Shuppan, authorized by Youichi Hayashida.)
The above pertains particularly to fuel injection control. A microcomputer calculates the most appropriate injection quantity in accordance with the program stored in memory in response to input signals derived from various types of sensors. The most appropriate injection quantity is injected into an intake manifold at the timing during which power is supplied to the solenoid coil of the fuel injection valve corresponding to the injection quantity.
The injection duration for normal engine operation is once per engine revolution for all cylinders, when fuel is injected through all fuel injection valves simultaneously. A reference position signal is produced from a crank angle sensor (120.degree. signal in the case of six-cylinder engine). In other words, a drive pulse is output to the injection valve at equal intervals for three inputs of the 120.degree. signal in the case of a six-cylinder engine.
The injection quantity of fuel includes a "basic injection quantity plus various correction quantities".
However, since fuel pressure acting on the fuel injection valves is held constant, the injection quantity corresponds to a pulsewidth supplied to each fuel injection valve during the time for which the fuel injection valve is opened. Therefore, an injection pulsewidth (Ti) when the engine operates normally is calculated using the following equation (1): EQU Ti=Tp.times.(1+K.sub.TW +K.sub.AS +K.sub.AI +K.sub.MR).times.K.sub.FC .times..alpha.+T.sub.S ( 1)
In the equation (1), a basic pulsewidth (T.sub.p) is a value (a quantity corresponding to a basic injection quantity) determined from an intake air quantity (Q.sub.a) and engine revolutional speed (Ne). The air/fuel mixture ratio determined by the basic pulsewidth T.sub.p is called a basic air/fuel mixture ratio. Values added to the equation (1) (correction coefficient of coolant temperature incremental quantity K.sub.TW, incremental correction coefficient K.sub.AS during start and after engine start, incremental correction coefficient K.sub.AI after engine idling, and air/fuel mixture ratio correction coefficient K.sub.MR) are coefficients to make corrections to the basic pulsewidth T.sub.p according to various kinds of engine operating conditions input from sensors other than an airflow meter (for example, K.sub.TW is introduced to enrich the air/fuel mixture along with a reduction in coolant temperature (T.sub.W) when it becomes effective below 60.degree. C., and a difference provided between the incremental correction quantities K.sub.TW depending on whether an idling contact is turned ON or OFF when it becomes effective above 10.degree. C.)
The total sum of these coefficients is equation (1) are expressed as various kinds of correction coefficients (Co). K.sub.FC denotes a fuel cut-off coefficient.
.alpha. denotes a feedback correction coefficient for the air/fuel mixture ratio and denotes a value at which three-element catalytic conversion (CCRO) functions efficiently. In order to clarify three components (CO, HC, No.sub.x) of exhaust gas by means of catalytic conversion (CCRO), the air/fuel (A/F) mix ratio of the air/fuel mix needs to fall within a limited range (this range or, so called window, has the stoichiometric air/fuel mixture ratio of air/fuel mixture as a center). Therefore, it is better to perform feedback control of the A/F ratio to achieve higher control accuracy.
FIG. 1 shows a program for calculating the feedback correction coefficient .alpha. described above executed in a previously proposed A/F ratio controlling system.
In FIG. 1, in a step S1, a CPU of the microcomputer starts the program and determines whether a control area of the A/F ratio falls within a feedback control area of the air fuel mixture ratio (for example, conditions such that the temperature of the air/fuel mixture ratio sensor increases above an active temperature and conditions indicating the engine has not been started nor been in the idle state are satisfied). (It is noted that in FIG. 1, this state is abbreviated as "F/B control area"). In the step S1, in a case where control does not fall in the feedback (F/B) control area, the routine goes to a step S15 in which .alpha. is clamped. The program shown in FIG. 1 is executed whenever the engine has rotated through a predetermined crank angle.
The program shown in FIG. 1 shows an example of a proportional-integration (P-I) control operation in which the control center of .alpha. is 1.0 and .alpha. periodically changes as shown in the lower stage of FIG. 2. According to the operation described above, one period is divided into four cases (i) through (iv).
(i) In a case where the air/fuel mixture ratio is inverted from lean to rich, the detected air/fuel mixture ratio is changed stepwise by a proportional portion (P.sub.R) to the lean side.
(ii) Thereafter, the air/fuel mixture ratio is gradually changed to the lean side by the integration portion (I.sub.R) during the continuation of the rich air/fuel mixture state.
(iii) In a case where the air/fuel mixture ratio is inverted from rich to lean, the air/fuel mixture ratio is changed stepwise by the proportional portion (P.sub.L) to the rich side.
(iv) Thereafter, the air/fuel mixture ratio is gradually changed to the rich side by the integration portion (I.sub.L) during the continuation of the lean A/F mixture.
The determinations to divide the air/fuel mixture ratio into the above-described four cases are carried out by a combination of magnitude comparisons between an output value of the air/fuel mixture ratio sensor and a reference level (corresponding to the sensor output value with respect to the stoichiometric air/fuel mixture ratio) in steps S2, S3, and S9 and thereof previously carried out. "RL" in steps S3 and S9 denote flags storing the previous results of magnitude comparisons. RL=R indicates that the A/F (air/fuel mixture) ratio has previously been at the rich side and RL=L indicates that the A/F ratio has previously been at the lean side. As a result of this, the routine shown in FIG. 1 goes to steps S2, S3, and S4 when the A/F ratio is changed from the lean side to the rich side. In the same way, the routine goes to the steps S2, S3, and S7 when the A/F ratio continues at the rich side. The routine goes to steps S2, S9, and S10 when the A/F ratio is changed from the rich side to the lean side. The routine goes to steps S2, S9, and S13 when the A/F ratio continues at the lean side. It is noted that immediately after the magnitude comparison is inverted, the flag is changed in value in steps S4 and S10 after the inversion of the A/F ratio is carried out.
The following equations (4), (5), (6), and (7) indicate the proportional portion (P.sub.R, P.sub.L) and the integration portion (I.sub.R and I.sub.L) according to the following cases in steps S5, S7, S11, and S13. EQU P.sub.R =K.sub.p .times.ERROR (4) EQU .SIGMA.I.sub.R =K.sub.I .times.ERROR (5) EQU P.sub.L =K.sub.p .times.ERROR (6) EQU .SIGMA.I.sub.L =K.sub.I .times.ERROR (7)
In the equations (4) through (7), ERROR denotes a difference from the stoichiometric air/fuel mixture ratio, K.sub.p and K.sub.I denote the feedback constant (K.sub.p denotes a proportional constant and K.sub.I denotes an integration constant). The same values as in the rich side and in the lean side can, in many cases, be adopted as shown in the equations (4) through (7). The feedback correction coefficients (.alpha.) use the proportional portion and integration portion in steps S6, S8, S12, and S14. In the case of (i), .alpha.=.alpha.-P.sub.R. In the case of (ii), .alpha.=.alpha.-I.sub.R. In the case of (iii), .alpha.=.alpha.+P.sub.L. In the case of (iv), .alpha.=.alpha.+I.sub.L. The meaning of these numerical equations is to be read as the value stored as .alpha.. The value added or substracted is newly added or subtracted as .alpha..
In such an apparatus as described above, the integration constant (K.sub.I) described above is a constant value determined according to engine revolutional speed, engine load, coolant temperature, and so on. Since a value which is different for steady-state driving and transient-state driving conditions is not adopted, a limit is generated when hunting occurs during normal driving states or when variation of the air/fuel mixture ratio cannot be absorbed during transient driving states.
For example, as shown in FIG. 2, the feedback correction coefficient .alpha. is changed in a case when a basic fuel/air mixture ratio (an inversion of the air/fuel mixture ratio) which provides integration for system error is substantially changed stepwise from the rich side to the lean side (during a transient state). The change pattern of .alpha. is such as to require a stepwise change corresponding to the change in the basic fuel/air mixture ratio, as shown by a dot-dash line in FIG. 2. That is to say, the dotted line shown in FIG. 2 gives the required value for .alpha..
However, since the actual value of .alpha. shown by a solid line is changed on the basis of the integration constant, a response delay occurs at an interval B shown in FIG. 2 with respect to the required value of .alpha.. This is because the integration constant defines a gradient for each line segment rising in a right-up direction or right-down direction. When the integration constant becomes large, the value of .alpha. is rapidly changed to enable improvement in response characteristics. When the control at the time of a steady state is performed with the integration constant having the same value as that during the transient driving state, hunting, in turn, occurs in the steady state. Therefore, the integration constant cannot be increased any more in that state.
In other words, since stability at the time of steady state driving conditions and good response at the time of transient state conditions are simultaneously required under such an air/fuel mixture ratio control, a value of the integration constant such as will not deviate from either of these requirements needs to be selected in order to balance these requirements with a single integration constant. A sufficient value thereof which simultaneously meets the above-described requirements is not always available under all engine operating conditions.